Book/Report

FZJ-2018-01560

http://join2-wiki.gsi.de/foswiki/pub/Main/Artwork/join2_logo100x88.png Messung der Elektronen Spinpolarisation bei der Photoemission an Nickel- und Kobalt-Einkristallen mit Photonenenergien in der Nähe der Photoschwelle

Kuhlmann, E. (Corresponding author)FZJ*

1980
Kernforschungsanlage Jülich, Verlag Jülich

Please use a persistent id in citations: http://hdl.handle.net/2128/17533

Report No.: Juel-1697

Abstract: We have measured with s-polarized light ne ar threshold the spinpolarization P of photoelectrons from Ni(001), -(110) and -(111) and from Co(0001)- and -(10$\overline{1}$0) surfaces. The cylindrically shaped samples were magnetized perpendicular to the surface (longitudinal magnetization) up to complete saturation by an external magnetic field. With these experimental conditions a differential energy- analysis as weIl as angle- analysis of the emitted electrons is not feasible. However, for photon energies 1n the range 1-2 eV above threshold the energy distribution of the collected electrons is narrow and, because of energy conservation, emission occurs primarily normal to the surface (cone<15$^\circ$). The effect of the external magnetic field on the electron trajectories and their spin direction has been determined by computational methods. As expected from the bandmodel of ferromagnetism (Stoner- Wohlfarth- Slater- theory) the experimental values found for the photoelectron polarization near photothreshold are strongly energy dependent. For Ni(001) and -(111) the dependence of spin- polarization on photon energy agrees quantitatively with theoretical calculations only if a value of 0.33 eV for the exchange splitting $\Delta_{ex}$ is assumed. This is in contrast to the results of self- consistent band structure calculations yielding values between 0.6 eV and 0.9 eV for $\Delta_{ex}$. These results provide clear evidence for the importance of many- body effects (electron- electron and electron- magnon interactions) in photoemission from strong ferromagnets. The same is true for the Stoner gaps along $\Gamma$x, $\Gamma$KX and $\Gamma$L, which are found to be $\delta(\Delta_{5}^{\uparrow} \rightarrow X_{5}^{\uparrow}) = \delta(\sum_{3}^{\uparrow} \rightarrow X_{5}^{\uparrow}$) $\simeq$ 0.02 eV and $\delta (L_{3}^{\uparrow}) \simeq$ eV, while calculations give values of $\delta (X_{5}^{\uparrow}) \geq$ 0.3 eV and $\delta(L_{3}^{\uparrow}) \geq$ 0.5 eV. Nickel differs therefore strongly from copper, where local density calculations of the ground state describesurprisingly accurately also the photoemission spectra. We compare the polarization data with results of energy- and angular- resolved photoemission (ARUPS-) data obtained by several other groups. This comparison shows extensive agreement concerning conclusions about the experimental electronic structure of Ni. Negative polarization near photothreshold has been observed, besides for Ni, also for Co(10$\overline{1}$0), but not for Co(0001) -- in contrast to expectations. The interpretation of these and other ARUPS- data of Co is still contradictory. The electron spin polarization has been measured as a function of temperature using a Ni(111) single crystal surface and constant wavelength of the incident light. The results indicate that the ESP-experiment tests the long- range magnetic order, while ARUPS seems to be measuring some sort of short- range order, the exact extent of which has still to be established. Finally, in an appendix we report studies of the emission current and electron polarization from a tungsten field- emission tip coated with europium- sulfide. It is shown that they give information on theinterface Properties metal- magnetic insulator as well as on the magnetic properties of EuS. Spin-filter- and "memory"- effects of the W/EuS system are discussed.

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Contributing Institute(s):

1. Publikationen vor 2000 (PRE-2000)

Research Program(s):

1. 899 - ohne Topic (POF3-899) (POF3-899)

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